High luster and clarity are highly desired in polyamide filaments to be used for certain styles of carpets. Clarity (of transparency) depends on the absence of particles or optical discontinuities within the polymer which refract or scatter light. Both clarity and high luster require a smooth filament surface lacking surface roughnesses which scatter or deflect incident light.
Furthermore, carpet yarns which can be dyed satisfactorily at room temperature are greatly desired to reduce the energy costs of finishing carpets whether or not the fibers also require high luster or clarity. The three properties may be desired individually or in various combinations in different carpet yarn products.
Room temperature dyeability was achieved in Kobsa and Windley, U.S. Pat. No. 4,559,196 by steam setting deep-dyeable salt blend copolymer of nylon 66 and nylon 6 wherein the nylon 6 constituted 6-12% of the material. However, a portion of the caprolactam vaporizes out of the hot filaments below the spinneret and condenses on the frame of the spinning machine or cold walls of the spinning enclosure causing deposits which can interfere with the filaments or the quench air flow. If the polymer is conditioned as flake, the caprolactam can deposit in the drying system ducts.
Most polyamides develop spherulites, which are regions within the filaments in which the molecular chains pack radially outward from a nucleus to form a spherical structure. Spherulites are undesirable both because they scatter light and because they impede the ordering of crystal structure into preferred alignments during drawing, resulting in brittleness or lower strength of the filaments. Large spherulites are generally more damaging to the physical properties and luster than small ones, so a conventional solution to spherulite problems is to add fine particulate matter of various sorts to act as nucleating agents, producing a large number of small spherulites rather than fewer large ones. However, the particulate matter and small spherulites make the polymer cloudy.